1. Field of the Invention
The present invention relates to an apparatus and method for forming a hemispherical microlens at the end of an optical fiber used for optical communication for the purpose of efficient optocoupling of an optical element and optical fiber.
2. Description of the Related Art
One known technique for efficiently optocoupling an optical fiber and an optical element involves forming the core portion of the end of an optical fiber as a hemisphere, and using the image magnification effected by the lens action of the curved surface to highly efficiently optocouple an optical element and optical fiber.
The method of forming the end of the optical fiber as a hemisphere, i.e., the hemispherical microlens forming method, involves first grinding the end of the optical fiber flat, heating and melting the end using a burner or electrical discharge heating, and forming a hemispherical shape by means of the surface tension of the melted fiber.
A hemispherical microlens forming method for forming only the core portion of an optical fiber as a hemisphere is disclosed in the author's patent application Japanese Patent Laid-open No. 224098/1993. In this method, the core portion of an optical fiber is caused, to protrude by etching the end surface of the fiber. The core portion is then heated, thereby forming the protruding core-as a hemisphere. In particular, this method enables a reduction of manufacturing costs when forming as hemispherical microlenses the ends of an optical fiber array, each to be optocoupled to an optical element of a plurality of arranged optical elements, because the hemispherical microlens forming method can be applied to the tip surfaces collectively of the plurality of optical fibers arranged as an array.
The hemispherical shape of the protruding core changes with the passage of heating time. According to the conventional method of determining heating time, an operator observes the tip shape while heating the fiber end and stops heating upon judging that the ideal shape has been achieved.
However, precise monitoring of the shape by the operator is complicated by the extremely small diameter of the hemispherical tip of the core, which is on the order of 10 micron. The resulting error in heating time causes disparity in the shape of the hemispherical tips of each optical fiber. This results in the disadvantage that there is variation in the optocoupling characteristic for the optical fibers and optical elements and a corresponding increase in optocoupling loss.
With the object of canceling this disparity in optocoupling characteristic between the optical fibers and optical elements, Japanese Patent Laid-open No. 188707/1990 discloses a scheme in which, as shown in FIG. 1, light is irradiated into one end of an optical fiber 3, the light emitted from the opposite end is cast upon a translucent screen 7, the change in the shape of the field pattern during heating is observed from the reverse side of this screen 7, and heating is stopped when the field pattern reaches a minimum size.
However, as shown in FIG. 2, due to the lens effect of the hemispherical tip, laser light emitted from the hemispherical core tip at the end surface of an optical fiber generally first converges and then diverges, spreading apart with distance. If the radius of curvature of the surface of the hemispherical tip of the optical fiber is small, the refractive effect of the lens increases and the far-field pattern (hereinafter abbreviated as "F.F.P.") grows. On the other hand, if the radius of curvature of the surface of the hemispherical tip is small, the image magnification increases, and as a result, the optocoupling loss between the optical fibers and optical elements diminishes. In other words, when the diameter of the irradiated pattern approximates a maximum, optocoupling loss is at a minimum.
The improvement shown by FIG. 1 means that, if it is assumed that the vicinity of the focal point of the lens formed by the hemispherical tip is the position at which the field pattern is smallest, the screen is positioned extremely close to the hemispherical tip of the optical fiber and variation is observed in the near-field pattern (hereinafter abbreviated as "N.F.P.") which is about the same size as the core diameter.
Accordingly, the size of the observed field pattern is at most approximately 1 mm, and the smallest point within its variation must be found to determine the time for stopping heating.